Rice polish is a by-product from milling rice, consisting of the inner bran layers of the kernel with part of the germ and a small portion of the starchy interior. Since the protein and other non-carbohydrate constituents, such as B vitamins, minerals and fat, are located in the outer peripheral portion of the rice kernel, rice polish is considerably more nutritious than milled rice. Rice polish contains approximately 10-12% protein and 12-15% fat, whereas milled rice contains approximately 6-7% protein and 1% fat.
Rice polish suitable for human use is difficult to obtain. The quantity of polish produced during the milling of rice is usually less than 1% of the total rice kernel. The fat content is very susceptible to both oxidative rancidity and hydrolytic rancidity that is produced by the lipase and lipoxidase enzymes in the polish. Although the rice polish when initially collected exhibits minimal fat deterioration, to prevent free fatty acid development it is normally stored at 28.degree. F. during shipment.
In spite of the care taken to protect the quality of rice polish, excessive free fatty acid development occasionally takes place. Such free fatty acids react with the starch and inhibit the action of diastatic enzyme systems during subsequent cereal processing to adversely affect quality, production rates and yield of such cereals. In addition, the resulting deterioration or oxidative rancidity imparts undesirable taste and texture characteristics to the final cereal product.
The development of free fatty acids in grains during storage has been studied by numerous investigators. Fatty acids are readily formed by lipase from the fats in grains, particularly under storage conditions of high temperature and high humidity. Molds, such as Aspergillus and Penicillum which produce lipase, will grow on grain and cause fatty acids to be produced from the grain.
While it is known that low temperature storage will retard free fatty acid development in rice polish, adequate cold storage facilities are not readily available in those areas where rice mills are usually situated. Another problem associated with the cold storage of rice polish is the relatively insignificant rate at which the rice polish is produced during milling. This necessitates the preliminary storage of polish under unfavorable temperature conditions until adequate quantities are collected to justify transportation to the cold storage area.
Controlled humidity storage in which the relative humidity in the storage area is maintained below about 40% to 50% has been investigated and found to also retard the formation of free fatty acids. However, in order to obtain maximum stabilization of the fat by this method, the free fatty acid content in the polish at the time the polish is introduced to the controlled humidity conditions must be very low.
Previous efforts to stabilize rice polish have involved inactivating the enzymes by infra-red heat. U.S. Pat. Nos. 2,585,978 and 2,610,914 are representative of patents disclosing such heat stabilization of rice bran, wheat bran and wheat germ. Although thermal inactivation of the lipase and lipoxidase enzymes inhibits the development of free fatty acids, frequently oxidative rancidity is enhanced due to the insufficient moisture in the material. As a consequence, prior art process techniques have not been widely practiced.
It is therefore an object of this invention to provide a method for stabilizing rice polish regardless of the section of the country in which it is produced, so that the rice polish is resistant to deterioration for exceptionally long periods of time.
Another object is to provide a method that is effective in resisting the formation of free fatty acid in rice polish that is adaptable for use at or near the place where rice polish is produced.
A still further object is to provide a method that is effective to inhibit oxidative rancity of rice polish.
Other objects, features and advantages of this invention will become apparent from a reading of this disclosure.